1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a tool for and a method of inspecting masks used in photolithography.
2. Discussion of Related Art
The manufacturing of integrated circuit (IC) devices involves the processing of a semiconductor wafer though sequential layers to add or remove material. Photolithography is used to pattern each layer on the wafer. The wafer is covered with photoresist which is a material that is sensitive to light. The photoresist is then selectively exposed with a mask in an exposure tool. An exposure tool may be a stepper, a scanner, or a contact printer. The photoresist is developed to form openings corresponding to the amount of light exposure. The photoresist acts as a stencil for transferring a pattern on the mask to the wafer through the process of etching.
IC manufacturing requires a total of 4 to 35 masks. A mask may be used for a critical layer of the IC such as isolation, gate, contact, or first metal. A mask may also be used for a non-critical layer requiring diffusion or ion implantation. The mask may be transmissive or reflective. The features may be magnified on the mask and then reduced back to the desired size by the exposure tool.
Conventional masks used in photolithography are made by patterning chrome that has been deposited on a quartz substrate. Such masks are binary since the amplitude of light is modulated by the opaque and transparent areas. However, when the minimum critical dimension (CD) becomes smaller than the exposure wavelength, the resolution of the images is degraded by the spreading effects of diffraction. Phase-shifting masks (PSMs) take advantage of destructive interference to minimize the detrimental effects of diffraction.
Systematic print biases and etch biases in the fabrication of wafers can affect image quality. Optical proximity effects are manifested as corner rounding, line shortening, and CD offset between nested features and isolated features. Optical proximity correction (OPC), based on models or rules, can be incorporated into binary masks and PSMs to compensate for repeatable image distortions. OPC involves the addition of sub-resolution features, such as serifs, assist slots, and scattering bars.
Masks using phase-shifting and OPC are complex and difficult to inspect. Inspection is necessary to ensure the fidelity of the masks. Patterns on the masks must meet stringent criteria for size, shape, spacing, orientation, overlap, and placement of features. Defects must be repaired to prevent replication of errors across the wafers.
Currently, masks are usually inspected with a scanning optical microscope. However, resolution is limited by wavelength of the illumination, numerical aperture of the condenser and collector lens, and aberrations in the optics. Phase information is lost since light intensity is integrated at each location on a mask.
Thus, what is needed is a tool for and a method of inspecting a mask used in photolithography to determine errors in phase, amplitude, and pattern edges with high resolution.